viable/strict/1787461123: Revert "Remove caffe2/core/timer.h (#194228)"

发生了什么

This reverts commit 7744b78 . Reverted #194228 on behalf of https://github.com/jeanschmidt due to internally many systems still depends on the deleted timer.h header, so a follow up is required to remove this dependency before merging this PR, @janeyx99 can you follow up on this one? - see D116988880 ( comment )

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